Star Trek - "Is There In Truth No Beauty?"
A solid third season TOS effort, thanks in large part to guest star Diana Muldaur as Dr. Miranda Jones. With her ice-blue eyes and measured tone of voice she's perfect as the conflicted telepathic aide to Ambassador Kollos, a Medusan* who is one of the most unusual aliens in sci-fi. Medusans are non-corporeal lifeforms that are encased in a special container and can't be viewed with the naked humanoid eye because to do so would drive a person insane.
Important safety tip: NEVER PEEK INTO A MEDUSAN FOOTLOCKER!
This episode packs a lot: we get to meet "one of the designers of the Enterprise" in Larry Marvick, we get to see all the male regulars stumble over each other while hitting on Dr. Jones, we go on another trip "where no man has gone before," Kirk gets shot down, Spock goes bananas, Dr. Jones has a personal crisis, Kirk gets to phaser his best friend, and we get some of the strangest photography of the series with the use of a fish-eye lens to convey the insanity Marvick and Spock feel after looking at Kollos.
Plus a few characters get to wear really cool red visors which they should have put on the market a long time ago for Trek fans. Maybe Ray Ban could make them.
Speaking of marketing, the Vulcan IDIC medallion makes its debut here. Gene Roddenberry's mail order company was about to put them out for sale and its appearance here would be a huge (and free) advertising gimmick for the jewelry. Allegedly it took some serious convincing to get Leonard Nimoy to wear the thing (it's unclear whether he got a piece of that action for being the IDIC pitchman).
* The one quibble with the Medusans is their name. It's beyond obvious that the name comes from the ancient Greek monster Medusa, who turned you into stone if you looked at her directly. Couldn't they have played with the name a bit so it wasn't so blatant? Oh, well....